Not all essential oils are equal. There is very little legistation with regard to what essential oils manufacturers have to put on the packaging. Many essential oils on the market today are diluted down and you wouldn't even know. To ensure you're buying premium quality here are a few tips.
1. Find out weather it's an established brand. Anyone can buy small(ish) amounts of cheap low quality essential oils from wholesalers with their own label on them and sell them from their website or ebay, and amazon etc.
2. Make sure the bottle has the latin name on it. This is the botanical name for the plant from where the oils is extracted. If you know this name you can find out the exact strain of the plant and discover how good it is.
3. Look to see if it says '100% pure' on the bottle, if not it's likely to be diluted down and not very good. It may small nice but it'll have no theraputic properties.